Digital method and apparatus for evaluating a frequency parameter of an if signal

Abstract

A tuning system of a television receiver includes a digital automatic fine tuning (AFT) unit for generating a digital AFT signal representing the deviation of the frequency of the picture carrier of the IF signal from a nominal frequency. The digital AFT unit includes a counter for counting cycles of picture carrier during repetitive counting intervals which occur during a measurement period having a duration at least as long as the duration of one field of a television signal. The duration and spacing of the counting intervals are selected so that at least one counting interval occurs during the vertical blanking interval in which the picture carrier is not likely to be over-modulated and therefore not likely to result in erroneous low counts. The contents of the counter are evaluated at the end of each counting cycle to determine if a first count corresponding to a nominal frequency or a second count corresponding to the frequency having a positive frequency deviation have been produced. At the end of the measurement interval, the frequency of the picture carrier relative to the nominal frequency is evaluated by determining which one, if any of the first and second counts have been produced. A low frequency is indicated by the absence of both. A frequency within specified limits is indicated by the presence of the first but not the second counts A high frequency is indicated by the presence of both counts. The technique employed does not require that the timing signal be synchronized with the vertical blanking interval for reliable counting.

I claim: 
     
       1. In a television receiver including a tuner for tuning an RF television signal having a picture carrier modulated with video information organized in repetitive fields including line intervals including image information and horizontal and vertical blanking intervals including synchronization information to produce an IF signal having a picture carrier corresponding to the picture carrier of the RF signal, wherein the tuner comprises: means for counting cycles of the picture carrier of the IF signal during repetitive counting intervals which occur during a measurement interval having a duration at least as long as the duration of one field;   means for generating a timing signal which defines the counting cycles as to duration and spacing so that at least one counting interval occurs within the vertical blanking interval; and   means for determining which one, if any, of first and second predetermined counts have been produced during the measurement interval to evaluate the deviation of the frequency of the IF picture carrier from a nominal frequency.   
     
     
       2. The apparatus defined in claim 1, wherein: the first predetermined count corresponds to a first range of frequencies surrounding the nominal frequency of the IF picture carrier and the second predetermined count corresponds to a second range of frequencies which are higher than and adjacent to the first range of frequencies.   
     
     
       3. The apparatus defined in claim 1, wherein: said means for determining which one, if any, of the first and second predetermined counts has been produced includes means operating under software program control for comparing counts produced during respective counting intervals with the first and second predetermined counts.   
     
     
       4. The apparatus defined in claim 3, wherein: said means operating under software program control a) after each counting interval, determines the count produced during the counting interval;   b) after each counting interval, stores the count produced during the counting interval; and   c) at the end of the measuring interval, evaluates the stored counts.   
     
     
       5. The apparatus defined in claim 3, wherein: said means operating under software program a) after each counting interval, determines the count produced during the counting interval;   b) after each counting interval, stores a first predetermined count indication if the first count has been produced during the counting interval and stores a second count indication if the second predetermined count has been produced during the counting interval; and   c) after the measuring interval, evaluates the stored count indications.   
     
     
       6. The apparatus defined in claim 3, wherein: said means for determining which one, if any, of the first and second predetermined counts has been produced includes first and second registers for storing respective first and second indications if said first and second counts, respectively, are produced.   
     
     
       7. A method of evaluating, with respect to a nominal frequency, the frequency of a picture carrier modulated with video information organized in respective fields including line intervals including image information and horizontal and vertical blanking intervals including synchronization information comprising the steps of: counting cycles of the picture carrier during repetitive counting intervals which are asynchronous with respect to the vertical blanking intervals and which have a duration and are spaced during a measurement period which has a duration at least as long as one field so that at least one counting cycle occurs within one of the vertical blanking intervals; and   determining, after the measurement period, which one, if any, of first and second predetermined counts have been produced during respective counting intervals.   
     
     
       8. The method defined in claim 7, wherein: the first predetermined count corresponds to a first range of frequencies surrounding the nominal frequency of the picture carrier and the second predetermined count corresponds to a second range of frequencies which are higher and adjacent to the first range of frequencies.   
     
     
       9. The method defined in claim 7, wherein: the determining step includes the steps of a) after each counting interval, determining the count produced during the counting interval;   b) after each counting interval, storing the count produced during the counting interval; and   c) after the measuring period, evaluating the stored counts.   
     
     
       10. The method defined in claim 7, wherein: the determining step includes the steps of: a) after each counting interval, determining the count produced during the counting interval;   b) after each counting interval, storing a first predetermined count has indication if the first count has been produced during the counting interval and storing a second predetermined count has indication if the second count has been produced during the counting interval; and   c) after the measuring period, evaluating the stored count indications.